I may have misinterpreted your post, I thought you were referring to the initial lane closure advance warning signs, rather than the signage at the point where the lane becomes closed. Obviously those who chose to continue into any closed section of road are idiots!
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It makes a lot of sense for customer service staff on platforms to be highly visible (hence giving them a 'hi viz'!) so that people can locate them when needed. It would be very difficult indeed to identify somebody wearing just a dark uniform on a packed rush hour platform full of commuters wearing similar clothing. When the info systems go down at a major station like Temple Meads, managers are sent out into the subways and such like to advise passengers, and they wear orange vests. If they didn't people would never find them amongst the crowds.
Regarding the first part of your post: Yes, quite, also my apologies for any misunderstanding. And the number of people driving down closed roads is getting to a very dangerous level - I speak from experience! Too many times people don't seem to expect a hazard on a closed road, after thinking that the signage is somehow irrelevant and driving down there anyway.
Regarding the second part: high-visibility clothing is one option for people giving out information, but another would be to have clearly non-high-visibility but nonetheless bright and distinctive uniforms. For example, similar to the GatEx staff uniform, which includes a red jacket, but perhaps brighter. For people who are just giving out information, the fact that the clothing is "distinctive" would be more of a priority, rather than "safe"! Naturally, work like despatch and so on may require GO/RT clothing, and technical work will require it, which is understandable. To take one example, I have been in a team marshalling people and vehicles at a certain large event (which I shall not name) where the black uniform t-shirts were simply printed with large, very distinctive text, thus easily identifiable to visitors, and EN 471 Class 3 clothing was used for traffic management only.